Semiconductor chips made in MOS (metal-oxide-semiconductor) technology often contain many devices in close proximity which produce or are affected by electrical or magnetic fields. This close proximity can result in unwanted interactions between the devices. To prevent these interactions, field oxides are grown "locally" between devices to isolate them from each other. However, these oxide isolation zones consume valuable space on the semiconductor which might otherwise be used for other devices. This is particularly true when these isolation zones include "bird's beak" formations. As explained below, bird's beaks are unwanted areas of field oxide at the edges of the isolation zones. As miniaturization increases, it becomes increasingly important to eliminate all field oxide which is not essential for isolating devices.
Field oxide growth is typically controlled by coating the silicon wafer upon which the oxide is to be grown with a silicon nitride film. The nitride film is then etched away where field oxide is desired. When the wafer is exposed to an oxygen source under oxidizing conditions, field oxide grows in the etched areas but not in the areas covered with the silicon nitride film. This results in local isolation regions.
The bird's beak problem occurs because silicon is normally covered with an oxide below the nitride. This "pad" oxide is typically in the range of 300-1000 Angstroms. As the field oxide grows, it grows into the pad oxide film beneath the silicon nitride film, due to the diffusion of oxidizing species into the sandwich formed by the pad oxide and the silicon nitride, forming a pointed structure which partially penetrates the area designated for devices in the upper surface of the substrate. This pointed structure is the bird's beak. It reduces available space on the wafer.
The early prior art for growing field oxides is reported in, for example, "Local Oxidation of Silicon and its Application in Semiconductor-Device Technology," J. A. Appels, E. Kooi, et. al., Vol. 25 of Philips Res. Repts, Pages 118-132 (1970). That article teaches the use of silicon nitride mentioned above. The standard local oxidation process begins with a silicon substrate 1 covered with a pad oxide 2, as shown in FIG. 1a. A film of silicon nitride 3 is then deposited over the pad oxide, as shown in FIG. 1b. This is commonly done in a reactor containing Silicon Hydride and NH.sub.3 (ammonia) supplied in a hydrogen carrier into the reactor which is maintained at 900.degree. C. to 1000.degree. C., a process known as chemical vapor deposition (CVD). After that, a SiO.sub.2 film 4 is formed over the silicon nitride 3, as shown in FIG. 1c. A photoresist layer may be employed instead of the masking SiO.sub.2 film 4 as is well known in the art. After photolithography, the SiO.sub.2 film 4 is etched in a buffered HF bath, producing the structure shown in FIG. 1d, and the silicon nitride 3 is etched in an H.sub.3 PO.sub.4 bath, producing the structure shown in FIG. 1e. As is well known in the art, H.sub.3 PO.sub.4 cannot be used if the photoresist layer is used in place of SiO.sub.2 film 4; conventional plasma etching with flourine chemistry is used to etch the photoresist layer.
The result (FIG. 1e) is a silicon substrate 1 masked by films of pad oxide 2, silicon nitride 3 and the SiO.sub.2 film 4. In those areas where field oxide is desired, the bare silicon (or the silicon substrate covered by native oxide) is exposed to an oxygen source. The masked wafer is placed in an oxidizing ambient, typically saturated with H.sub.2 O at 1000.degree. C. for 6 to 15 hours to grow the field oxide 5, producing the structure shown in FIG. 1f. Finally, acid baths are again used to remove the remaining films of silicon dioxide 4, silicon nitride 3 and pad oxide 2, producing the structure shown in FIG. 1g. Prior to the formation of the field oxide 5, the SiO.sub.2 layer 4 is often removed after forming masks of silicon nitride 3. Direct deposition of nitride on native oxide give rise to stress induced leakage. In the existing art, the native oxide, which is less than 40 Angstroms, is replaced by a grown "pad" oxide to relieve the above stress. This "pad" oxide is in the 300-1000 Angstrom range.
It was with the use of these early processes that bird's beak problems developed as explained above (see, e.g., FIG. 1f). The Appels, et al. article does not teach any method for limiting bird's beak growth. Various methods in the prior art attempt to limit the growth of bird's beak.
One prior art method for reducing bird's beak (reported in "Sealed Interface Local Oxidation Technology, " J. Hui, et. al., IEEE Transactions on Electron Devices, Vol. ED-29, No. 4, pages 554-561 (1982) ) teaches the addition of nitrogen into the native oxide (before the silicon nitride masking film is formed) by either nitrogen ion implantation (low energy) or plasma-enhanced thermal (high temperature) nitridation. A silicon nitride layer is then formed on top of the nitridized native oxide and the process continues in the normal manner. Another prior art technique is reported in "A Bird's Beak Free Local Oxidation Technology Feasible for VLSI Circuits Fabrication," K. Y. Chiu, et. al., IEEE Transactions on Electron Devices, Vol. ED-29, No. 4, Pages 536-540 (April 1982). That technique starts with a silicon substrate 1 and a stress relief oxide 6 shown in FIG. 2a, which is masked with silicon nitride 3 and then etched (in several masking and etching steps, the Si.sub.3 N.sub.4, SiO.sub.2 and Si substrate are etched) in the areas where field oxide is desired, resulting in grooves in the silicon base, as shown in FIG. 2b. A second stress relief oxide 7 is formed on the grooves and sidewalls of the newly etched surface, as shown in FIG. 2c. A second film of silicon nitride 8 is then deposited on the entire surface as shown in FIG. 2d.
The structure shown in FIG. 2d includes the elevated island region, where devices (e.g. a MOSFET--an MOS Field Effect Device) will be applied and the planar surface of the grooves where the field oxide will be formed. In FIG. 2d, the island is covered by both nitride 3 and nitride 8 while the planar surface of the grooves is covered by nitride 8. A strongly undirectional plasma nitride etch is applied to the structure of FIG. 2 d; to remove the nitride 8 on the surface of the grooves such that the sidewalls of the island remain covered by nitride 8 and the top surface of the island is covered by the nitride 3, resulting in the structure shown in FIG. 2e (Note: the stress relief oxide 7 may actually remain on the planar surface, but it is not shown in FIG. 2e).
When the field oxide 5 is grown, birds' beaks are prevented by the sidewall masking effect of the nitride 8; see FIG. 2f. The silicon is then cleaned of remaining films above the device region and the field oxide 5 is etched before semiconductor devices are applied, as shown in FIG. 2g.
Another prior art technique for reducing bird's beak uses a nitrided oxide as a local oxidation mask and the final gate dielectric. See "A Metal-Gate Self-Aligned MOSFET Using Nitride Oxide", M. A. Schmidt, e. al., IEEE Transactions on Electron Devices, Vol. ED-32, No. 3., pages 643-648 (Mar. 1985). The technique produces a nitrided oxide, used as a field oxide mask, as the source/drain implant mask and as the gate dielectric, by a long exposure (2 hours) at 1000.degree. C. in a pure ammonia ambient. After forming the nitrided oxide, a photoresist layer is deposited and patterned by etching to produce a photoresist mask over the channel region. The photoresist (and nitrided oxide) mask a source-drain implant. The nitrided oxide is etched after the implant and the photoresist is removed, leaving the nitride oxide only in the channel region. Field oxide is now grown.
Some of these prior art techniques are illustrated in FIGS. 1a through 2g, which represent the prior art. As will be understood by those in the art, the FIGS. 1a through 3f illustrate one device region which is usually surrounded by many such similar device regions.
The prior art techniques for reducing bird's beak increase the number of steps for and the complexity of field oxide growth over the standard local oxidation process. Further, these processes, using high temperature nitridization steps, are incompatible with large wafer (e.g. 5 inch diameter wafers), high density CMOS production. For example, because of the long exposure to high temperatures, long thermal nitridizations can warp large wafers. It can also propagate subsurface defects (in, for example, defect free denuded zones) and deteriorate subsurface implants. In CMOS, the PMOS device design often incorporates a N-type retrograde implant early in the process. This retrograde profile needs to be very shallow to prevent punch through. A long, high temperature step will drive (through diffusion) the implant deep. Wet acid etching has drawbacks as well. For nitridized films the effectiveness of such etching varies with film thickness and film nitridization. As a result, large erratic encroachments of bird's beak develop where there are thin spots in the film.
The present invention overcomes these shortcomings in the prior art without increasing the complexity of the process or the number of steps necessary to manufacture semiconductor integrated circuit chips. In the sections below a new process is described for local oxidation of silicon which minimizes bird's beak.